1. Field of the Invention
The present invention relates to a thin film transistor and a method for producing the same.
2. Description of the Related Art
Recently, much attention is paid on a liquid crystal device with a constitution comprising thin film transistors (TFTs). In a device of this type denoted as an active matrix type liquid crystal display device, a TFT is formed in each of several million pixels or more arranged in matrix and the charge stored in each pixel is controlled by the TFT. Since a liquid crystal display device of the active matrix type enables a fine display at a high speed, it is utilized in a display device of a hand-held word processor or a computer.
Generally, at present, in an active matrix type liquid crystal display device, the TFTs provided in each pixel is produced using an amorphous silicon film formed by plasma CVD, and a peripheral driver circuit for driving the TFTs provided to each pixel is constructed by an external IC. The peripheral driver circuit is constructed by an external IC because the operation speed of a TFT using amorphous silicon film is too slow to satisfactorily operate as a peripheral driver circuit. In addition, although the peripheral driver circuit is generally constructed by a CMOS circuit, there is a problem that a CMOS circuit cannot be fabricated, because the properties of a P-channel TFT based on an amorphous silicon film are too inferior as compared with those of an N-channel TFT.
A TFT based on an amorphous silicon film is utilized in a liquid crystal display device of an active matrix type because the heat resistance must be considered in case a glass substrate is used. In general, a liquid crystal display device requires a transparent substrate. Thus, a type of material of the substrate is limited. Generally, an inexpensive transparent material available as a large-area substrate is confined to glass. However, since distinct shrinking and warping appear on a glass substrate when it is heated at 600.degree. C. or higher, a glass substrate can not be used practically in a process having heating treatment at 600.degree. C. or higher. For example, the deformation (strain) temperature of a Corning 7059 glass substrate that is commonly used as a substrate for a liquid crystal display device of an active matrix type is 593.degree. C. Thus, if heating treatment at a temperature higher than the deformation temperature is performed for the Corning 7059 glass substrate, a large warping or shrinking occurs on the substrate to make it practically unapplicable.
On the other hand, an amorphous silicon film can be formed easily over a large area at a low temperature (400.degree. C. or lower) by plasma CVD.
As described above, in a conventional technique, when a glass substrate is used, a semiconductor portion of a TFT to be produced may be constructed by an amorphous silicon film.
An active matrix type liquid crystal display device using quartz substrate is also known. Since a device of this type allows a heating treatment at 800.degree. C. or higher, or 900.degree. C. or higher, a TFT using a crystalline silicon thin film can be produced. Since a TFT based on a crystalline silicon film can be operated at a speed far higher than that of a TFT based on an amorphous silicon film, a finer display can be realized at a higher speed. Also, by using a crystalline silicon film for the TFT, a peripheral driver circuit can be arranged on the same substrate (a quartz substrate) to realize a compact and light-weight liquid crystal display device.
However, a quartz substrate is very expensive, and it costs 10 times as high as the price of a glass substrate. Thus, a quartz substrate is economically unfeasible.